1. Field of the Invention
The present invention relates to a light-and-shade inspection apparatus and method for use in inspection of light-and-shade defects on a plain material roll (or web) based on an image data which is produced by an image pick-up device picking up an image of the web (e.g., paper, film, nonwoven fabric, etc.) having a certain width and traveling in one direction. More particularly, the invention relates to a light-and-shade inspection apparatus for inspecting light-and-shade portions and a method for implementing it, by which variations of a background brightness (e.g., luminance) occurring in a width (e.g., transverse) direction of the web can be accurately eliminated.
2. Description of the Related Art
FIG. 10 is a block diagram showing a conventional light-and-shade inspection apparatus 1000 used for inspecting a web 2. Apparatus 1000 includes a line-sensor camera 1 for picking up an image of the web 2 as an inspected object having a constant width and traveling in one direction, an illuminator 3 for illuminating a zone picked up by the camera 1, and an image processing device 4 for processing data of an image picked up by the camera 1 and inspecting light and shade defects on the web.
The line-sensor camera 1 includes, for example, a photodetector array of 1024 elements (e.g., a charge-coupled device (CCD)) disposed along a line. Specifically, the camera 1 is disposed upwardly of a central portion of the web so that the photodetectors are arranged in a line array across the web in a transverse (width) direction thereof and in parallel with the transverse direction. The illuminator 3 is disposed downwardly of the web 2 (e.g., on the underside of the web), so as to illuminate the zone to be picked up by the camera 1 from a rear surface of the web 2.
The image processing device 4 includes an image input portion 5 for performing an analog-to-digital (A/D)-conversion of a picked-up image signal outputted from the camera 1 and for capturing into the image processing device 4 the resultant (digitized) signal as an image data of luminance (e.g., lightness or brightness) information (e.g., specified with 256 levels of gray), a first comparator 6 coupled to an output of the image input portion 5 and adapted for comparing the image data with a first threshold value S1, and a second comparator 7 also coupled to the output of the image input portion 5 and adapted for comparing the image data with a second threshold value S2. The first threshold value S1 is preset in a first preset device 8 and the second threshold value S2 is preset in a second preset device 9.
Referring to FIG. 11, an operation of the above-mentioned light-and-shade defect inspection apparatus 1000 will be described. In FIG. 11, a time-chart shows a lightness pattern of the image data obtained with one scan of the line-sensor camera 1. In the time chart of FIG. 11, the horizontal axis (x) indicates a position on the web 2 in the transverse direction, whereas the vertical axis indicates a degree of lightness of the image data.
If the web 2 has thereon a defect which tends to increase a lightness (e.g., a hole or a flaw), there appears in the image data a high lightness portion (e.g., reference numeral 11 or 12), as shown in FIG. 11. The comparator 6 detects a light defect when this portion is higher than the first threshold value S1. On the other hand, if the web 2 has thereon a defect which tends to lower its lightness (e.g., caused by a stain, adhered foreign matter, etc.), there appears in the image data a low lightness portion (or a shade portion) indicated by numeral 13 or 14, as shown in FIG. 11. The comparator 7 detects a shade defect when this portion is lower than the second threshold value S2.
Thus, the conventional light-and-shade defect inspection apparatus 1000 inspects the presence or absence of light-and-shade defects by comparing the image data with two threshold values.
However, when the line-sensor camera 1 picks up an image of the web 2 while the web 2 is illuminated by the illuminator 3 as shown in FIG. 10, the lightness of the web 2 tends to increase at central and neighboring portions thereof due to characteristics of camera lenses and a condensing degree of the illuminator. As a result, the obtained image data of the picked-up image may vary, as shown in FIG. 12 rather than that of FIG. 11. In that case, if threshold values for the image data of FIG. 12 are fixed as S1, S2 shown in FIG. 11, defects such as numerals 11A, 13A of FIG. 12 may not be detected, despite being detected correctly as true defects, since they are apparently smaller in magnitude or size than their corresponding threshold values.
In view of the foregoing, a light-and-shade defect inspection apparatus 1300 as shown in FIG. 13 is known to eliminate the above-mentioned problem.
Referring to FIG. 13 wherein like reference numerals of FIG. 10 refer to similar parts, an image processing device 4A is shown.
Image processing device 4A includes an image input portion 5 for performing an A/D-conversion of a picked-up image signal outputted from the camera 1 and for capturing the resultant (digitized) signal as an image data into the image processing device 4A, a one-dimensional filter 15, coupled to an output of the image input portion 5, for performing a smoothing processing of 1024 image data obtained by one scan of the camera 1, a first adder 16, coupled to an output of the filter 15, for obtaining a first threshold value S1xe2x80x2 by adding a first preset value to a smoothed signal outputted from the filter 15, and a second adder 17, similarly coupled to the filter 15, for obtaining a second threshold value S2xe2x80x2 by adding a second preset value to the smoothed signal.
Furthermore, the image processing device 4A includes a first comparator 18, coupled to outputs of the image input portion 5 and the first adder 16, for comparing the image data with the first threshold value S1xe2x80x2, and a second comparator 19, coupled to outputs of the image input portion 5 and the second adder 17, for comparing the image data with the second threshold value S2xe2x80x2. Here, the first threshold value S1 is preset in a first preset device 20 and the second threshold value S2 is preset in a second preset device 21.
With this configuration, the first and second preset values in the first and second preset devices 20 and 21 are respectively added to the smoothed signal obtained by the one-dimensional filter 15, thereby allowing the respective preset values S1xe2x80x2 and S2xe2x80x2 to be changed in response to the web""s lightness (luminance) variation across the width thereof. Hence, the defect 11A and 13A, which is not detected as defects in FIG. 12, can be detected as shown in FIG. 14.
However, despite the improved apparatus, a problem associated with the known improved light-and-shade defect inspection apparatus 1300 resides in that an area of each defect shown in FIG. 11 or 14 increases. It is noted that a length L of the defect appearing as an image data may be equal to or longer than an average length of data outputted from the one-dimensional filter 15. In that case, a lightness variation due to the defect may be processed similarly to a lightness variation of the background lightness, and the threshold value will vary as the defect lightness. As a result, the calculation of the background lightness is affected by the defect lightness, thereby causing an error in a calculated value.
For example, a problem occur that only portions W2, W3 could be detected as being defective within a zone W1 which zone should be essentially detected as a defect but a central portion W4 might not be detected as being defective.
The above problem is caused because an image pick-up range across the width of the web covered by the line-sensor camera is restricted, and the area of the defect increases with respect to the average length of data used in the image picked-up range. Also, this problem becomes worse as the defective area becomes larger.
Accordingly, the conventional light-and-shade defect inspection apparatus 130 possesses a significant deficiency that a light-and-shade defect inspection of a web having thereon various size defects (e.g., ranging from a small defective area to a large defective area), cannot be performed by using only one optical system.
In view of the foregoing and other problems of the conventional systems and methods, the present invention is directed to a light-and-shade inspection apparatus and method with a high reliability and low cost. With the present invention, defects on a web can be inspected reliably by using only one optical system and by accurately eliminating the background lightness regardless of a size of the defects (e.g., ranging from a small area to a large area). As a result, a light-and-shade inspection is provided at a low manufacturing cost and with a high accuracy.
To overcome the above-mentioned problems, in a first aspect of the present invention, a light-and-shade inspection apparatus is provided for picking up an image of an object to be inspected having a constant width and traveling in one direction to produce an image data and then performing a light-and-shade inspection of the object based on the image data, and which includes:
an image pick-up device for picking up an image of an object across a overall width thereof to produce an image data;
a projection operational portion for calculating a projection data by adding together the image data by a predetermined number, the image data being produced at each scan of the image pick-up device at a predetermined position along the width of the object;
a background lightness operational portion for calculating a background lightness of the object across its width based on the projection data produced by the projection operational portion; and
a difference operational portion for subtracting the background lightness produced by the background lightness operational portion from the image data produced by the image pick-up device, thereby eliminating variations of the background lightness across the width of the object from the image data.
With this arrangement, an amount of additional data can be freely increased for the calculation of the projection data in response to the number of scans. As a result, the number of data used in calculating the background lightness can be increased substantially in comparison with the number of data for the defective area. Thus, the present invention reduces the adverse affects of defects from the calculation of the background lightness, thereby removing variations of the background lightness from the image data with high accuracy.
Additionally, in the light-and-shade inspection apparatus according to the present invention, the background lightness operational portion includes:
a filter for smoothing the projection data across the width of the object produced by the projection operational portion; and
a normalization portion for dividing the smoothed data by the predetermined number of the added image data, thereby calculating the background lightness.
Further, the light-and-shade inspection apparatus according to the present invention further includes a comparator portion adapted for comparing an output from the difference operational portion with a predetermined value to detect light-and-shade defects.
Hence, a highly reliable light-and-shade inspection apparatus is provided capable of detecting a defect having a certain lightness regardless of the defect""s size and with high accuracy.
Furthermore, in the light-and-shade inspection apparatus according to the present invention, the image pick-up device includes a line-sensor camera directed to the object and arranged across the width of the object. Even if such a line-sensor camera is employed, the amount of data for the background lightness calculation can be increased in response to the number of scans of the camera, thereby allowing the background lightness to be calculated with a high accuracy. Also, the use of this line-sensor camera allows the manufacturing cost to be reduced in comparison with the use of an area-sensor camera.
Furthermore, in the light-and-shade inspection apparatus according to the present invention, the predetermined number of the image data to be added together at the projection operational portion can be changed in response to a light-and-shade defective area size.
With this arrangement, for a large defective area, the number of the image data to be added together is correspondingly increased and, for a small defective area, that number is correspondingly decreased, thereby allowing the background lightness to be obtained with only one optical system with high accuracy and efficiency and without any affect of the defective area size. Also, this arrangement is inexpensive. Incidentally, changing the predetermined number is performed by operating and varying a preset number N in the projection operational portion.
Furthermore, in the light-and-shade inspection apparatus according to the present invention, the background lightness operational calculation at the background lightness operational portion is performed at the completion of a predetermined number of scans by the image pick-up device, and the difference operational calculation at the difference operational portion is performed with respect to an inputted image data by using the background lightness calculated through the background operational calculation which has been performed immediately prior to that input of the image data.
With this arrangement, an accurate background lightness may be produced by following variations of the background lightness occurring during movements of the object or over time, and further accuracy, reliability and stability of the background lightness can be ensured.
In another aspect of the invention, a light-and-shade inspection method according to the present invention, wherein an object to be inspected having a constant width and traveling in one direction is picked up to produce an image data thereof and then inspecting based on the image data light-and-shade portions on the object, includes:
picking up an image of an object across a width thereof to produce an image data;
calculating a projection data by adding together the image data by a predetermined number, each image data being produced at a predetermined position along the width of the object;
calculating a background lightness of the object across the width by performing smoothing and normalizing processing operations of the projection data; and
eliminating variations of the background lightness across the width of the object from the image data by subtracting the calculated background lightness from the image data.
By implementing this method, an additional amount of data can be increased freely for calculating the projection data in response to the number of scans. As a result, the number of data for calculation of the background lightness can be increased substantially in comparison with the number of data for the defective area. Thus, adverse affects of defects are reduced from the calculation of the background lightness, thereby removing variations of the background lightness from the image data with high accuracy.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-205251, filed Jul. 21, 1998, which is expressly incorporated herein by reference in its entirety.